The subject matter disclosed herein relates to power converters and, more specifically, to improved control of and improved power conversion from polyphase alternating current (AC) machines during low speed operation.
In recent years, increased demands for energy and increased concerns about supplies of fossil fuels and their corresponding pollution have led to an increased interest in renewable energy sources. Two of the most common and best developed renewable energy sources are photovoltaic energy and wind energy. Other renewable energy sources may include fuel cells, hydroelectric energy, tidal energy, and biofuel or biomass generators. However, using renewable energy sources to generate electrical energy presents a new set of challenges.
Wind turbines, for example, provide a variable supply of energy. The supply is dependent on the amount of wind. Wind turbines are typically configured to generate AC energy and typically provide a multi-phase AC voltage at varying current levels. Due to the variable nature of the energy supplied, power converters are commonly inserted between the wind turbine and the utility gird or an electrical load, if operating independently of the utility grid. The power converters typically require that the wind turbine be rotating at a minimum speed, also known as a cut-in speed, such that it is generating a minimum level of power before the power converter begins operation.
However, wind turbines have substantial mass and require significant energy to accelerate from a stop to the cut-in speed such that the converter may begin to harvest energy. Further, some wind turbines may have an inertial “knee”, meaning they require a greater amount of energy to overcome, for example, static friction forces and begin rotation than the amount of energy required to continue rotation of the turbine. The inertial “knee” may, therefore, require a higher initial wind speed to begin operation of the wind turbine, but once the initial speed has been obtained, operation could continue at lower wind speeds. As a result, the wind turbine may have a range of wind speeds at which it may be capable of generating energy but the energy is lost if the wind turbine was not already rotating. Similarly, the inertia of a wind turbine may cause slow acceleration from a stop even if the wind is strong enough to accelerate the turbine to the cut-in speed. The slow acceleration may result in an undesirable amount of time to accelerate the wind turbine up to the cut-in speed. During the acceleration, the wind turbine is again failing to produce energy during periods at which the wind speed is sufficient for energy generation.
In order to obtain the highest potential energy generation from the wind turbine, it is desirable to have the wind turbine operating above the cut-in speed of the converter as often as possible. Thus, it would be desirable to provide a system that can overcome the inertial “knee” and/or help accelerate the wind turbine up to the cut-in speed.